chances are that only one of these patients will be confined upon the
day predicted; nine will be confined before and ten after it. In all
probability five of those who pass the predicted day will be
delivered within a week and four others within the second week, while
the twentieth patient will not be delivered until three weeks or more
have elapsed.

Such results clearly indicate our inability to make accurate
predictions even though pregnancy is normal in every way. Whenever
patients pass their expected date uneventfully, if they will bear in
mind that the fault lies with the method of prediction and not with
the pregnancy, they will often be saved anxiety. Frequently such
discrepancies are attributable to a false assumption, for our rule
always assumes that the conception took place immediately after a
menstrual period. While this is generally true, the number of cases
in which it occurs just before the period to be missed is by no means
inconsiderable, and in these we should not expect pregnancy to end
until two or three weeks after the day predicted by the rule.

Occasionally patients know the precise day upon which conception took
place, and prefer to estimate the day of confinement from that rather
than from the beginning of the last menstruation. They may do so by
counting back thirteen weeks from the day of conception; but this
method also is subject to error for, as we have noted, the duration
of pregnancy reckoned in this more exact manner is not constant. Such
a calculation rarely offers any advantage over that made from the
menstrual record.

Another method of estimating the date of confinement is based upon
the assumption that fetal movements are first perceived by the mother